1. Field of the Invention
The present invention is related to a technique for double-sided printing of plural pages.
2. Description of the Related Art
In recent years, due to progress in various techniques, image forming apparatuses comprising various functions are being provided. Also, the environment in which image forming apparatuses perform printing is not only an environment where print driver software that corresponds to the image forming apparatus is installed on a client PC for printing, but rather environments have become diversified. For example, there are also environments wherein a host server is controlled from a terminal, and printing is performed via an IPDS protocol via an application on the host server (IPDS is a trademark of IBM). In order to support this kind of environment, there exists a configuration for connecting a host server with an image forming apparatus via an external controller. In this configuration, communication is performed between the host server and the external controller by the IPDS protocol, and communication is performed between the external controller and an image forming apparatus by a protocol that the image forming apparatus supports. In other words, the external controller realizes printing by bidirectionally converting the IPDS protocol to a protocol that the image forming apparatus supports (for example, refer to Japanese Patent Laid-Open No. 2006-178608).
Additionally, by extending a protocol of an image forming apparatus, an industry standard protocol such as IPDS may also be supported. There is a need for a guarantee of an operation according to a standard protocol instruction on the image forming apparatus side according to the type of the instruction. For example, for an external controller and an image forming apparatus, it is necessary to wait for continuing data or an instruction after all image data that is received according to an SRP command of IPDS is printed. For this reason, the external controller controls, for example, an image forming apparatus by transmitting a command (hereinafter referred to as PRP (PrintReceivedPages)) to instruct printing of all received image data. Once the image forming apparatus receives the PRP instruction, and prints all the received image data, it waits for continuing data or a continuing instruction to arrive. Since there is the possibility that continuing data may arrive, for example, even if it is a job for which a designated is made for post-processing such as stapling, the post-processing itself is not executed at that point in time, and preparation is made so that the continuing data can also be post-processed.
However, in a case where a PRP instruction is received in a state in which up until an odd page in a double-sided printing job are input into an image forming apparatus, and the received pages are printed, the odd pages are printed on the front-side while the even pages are printed on the back-side, and printing is performed only on the front-side of the last sheet. Then, the continuing pages that are input afterwards are printed from the front-side of a new sheet. In other words, for the continuing portion, the even pages are printed on the front-side, and the odd pages are printed on the back-side, and there is the possibility that a different printed material than the user intended will be generated. Normally, a host server and an external controller are made such that there is no case where a PRP instruction is received after receiving an odd page in this kind of double-sided printing job. However, when the host server and external controller fall into an unexpected state, in this kind of abnormality, there may be a possibility that a PRP instruction is issued at an unexpected timing. In this case, there is a possibility that a printed material that is not what the user intended will be generated, and that printed sheets will be wasted.